1. Field of the Invention
The present invention relates to techniques for converting color coefficients between color spaces based on color profiles.
2. Related Art
Color-management systems (CMSs) attempt to ensure that color values that are displayed on one output device appear the same when the color values are displayed on a different output device. To accomplish this, CMSs use color profiles, which describe how a given output device reproduces color, to ensure that color values are properly converted for a specific output device. Given two color profiles for different output devices, a set of color coefficients that represent a given color value on one output device can be converted to a corresponding set of color coefficients that represent the same color value on another output device.
Color values are defined with reference to “color spaces,” such as XYZ, LAB, RGB (red, green, blue), and CMYK (cyan, magenta, yellow, key/black). Different color spaces are used for different applications. For example, CMYK is typically used for print applications, whereas RGB is typically used for displaying colors on a computer display.
When displaying or printing CMYK colors on a non-CMYK device (such as an RGB display, an RGB printer, or a grey printer) combinations of color profiles are typically used to convert colors coefficients in the CMYK color space to color coefficients in the destination color space (i.e. RGB or grey).
However, color profiles are commonly-used to convert CMYK black (K=100%) to a color value other than RGB black (R=G=B=0), which can have undesirable consequences in some cases. For example, consider a CMYK document created for a newspaper press. Black text within the CMYK document is actually printed as a shade of grey on newsprint. On the other hand, in order to display the black newspaper text on an RGB display, the CMYK color coefficients for the black newspaper text are first converted into RGB color coefficients. However, this conversion process causes the black newspaper text to be displayed as a shade of grey on the computer display. Hence, a user viewing the text on the computer display does not know whether the text will be printed as black or as grey.
There are several existing techniques for converting CMYK colors into RGB equivalents:                (1) Conversions using the process described in PostScript Language Reference, Third Edition, p. 477 (Section 7.2.4 Conversion from DeviceCMYK to DeviceRGB), which use simple mathematical relationships to map between R, G, B color coefficients and C, M, Y, K color coefficients (see Adobe Systems Incorporated. PostScript Language Reference, Third Edition. Reading: Addison-Wesley Publishing Company, 1999.);        (2) a combination of two color profiles; the first profile (e.g. CMYK) is used to produce color coefficients for the color values in a “device-independent” color space (e.g. LAB or XYZ), which are then processed using the second color profile (e.g. RGB) to obtain equivalent RGB color coefficients. Typically, a combination of matrices and lookup tables are used for these conversions; or        (3) device link profiles, which perform a direct mapping from CMYK to RGB using a single lookup table.        Unfortunately, none of the above techniques produce idealized black conversions without problems.        (1) The conversion process described in PostScript Language Reference, Third Edition, p. 477, produces idealized black, but generates unacceptably inaccurate representations of most other CMYK colors.        (2) Combinations of color profiles allow for high-quality color transformations, but even with black-point compensation, input color values with K=100% do not automatically map to RGB black. Note that black-point compensation adjusts CMYK to RGB color conversions so that the darkest CMYK color value (C=M=Y=K=100%) maps to the darkest RGB color value (R=G=B=0). In order to achieve idealized black conversion, a special CMYK profile must be specially constructed to take advantage of black-point compensation to achieve idealized black. Unfortunately, because the CMYK profile must be specially constructed, the flexibility of the International Color Consortium (ICC) framework is lost.        (3) Device link profiles can achieve idealized black conversions, but unfortunately, these profiles have a number of problems: the profiles are not common; the software to create them is complicated and/or expensive; and few applications support them.        
Another technique is to treat K=100% as a special case and map it to RGB or grey black. However, this results in a sudden transition from grey to black as the K-channel color coefficient goes from 99% to 100%. Furthermore, this technique can only be applied to the special case where there are “solid” areas of color, not gradients or images. Otherwise, a black endpoint of a gradient may not match the color of adjacent black text.
Hence, what is needed is a method and an apparatus for providing a idealized black conversion of CMYK color values without the problems described above.